<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        div {
            width: 100px;
            height: 30px;
            border: 1px solid black;
            border-radius: 4px;
            margin: 100px auto;
            font-size: 16px;
            text-align: center;
            line-height: 30px;

        }
        ul {
            display: none;
            height: 140px;
        }
        ul li {
            list-style: none;
            height: 30px;
            line-height: 30px;
            border-bottom: 4px solid red;
        }
    </style>
    <script src="jquery-3.2.1.min.js"></script>
    <script>
        $(function () {
            $('div').hover(function () {
                $('ul').slideDown(1000);
            },function () {
                $('ul').slideUp(1000);
            })
        })
    </script>
</head>
<body>
<div>
    我是个导航
    <ul>
        <li>菜单1</li>
        <li>菜单2</li>
        <li>菜单3</li>
        <li>菜单4</li>
    </ul>
</div>
</body>
</html>